Leetcode 232. Implement Queue using Stacks

原题：
 
Implement the following operations of a queue using stacks.
push(x) -- Push element x to the back of queue.
pop() -- Removes the element from in front of queue.
peek() -- Get the front element.
empty() -- Return whether the queue is empty.
Notes:
You must use only standard operations of a stack -- which means only

push to top

,

peek/pop from top

,

size

, and

is empty

operations are valid.
Depending on your language, stack may not be supported natively. You may simulate a stack by using a list or deque (double-ended queue), as long as you use only standard operations of a stack.
You may assume that all operations are valid (for example, no pop or peek operations will be called on an empty queue).
 
解决方法：
 关键还是在push函数，弹出数直到stack为空。加入新的数，原来的数反序加入。

 
代码：
 class MyQueue {
stack<int> s;
public:
/** Initialize your data structure here. */
MyQueue() {

}

/** Push element x to the back of queue. */
void push(int x) {
if (s.size() == 0){
s.push(x);
}
else{
int val = pop();
push(x);
s.push(val);
}
}

/** Removes the element from in front of queue and returns that element. */
int pop() {
int val = s.top();
s.pop();
return val;
}

/** Get the front element. */
int peek() {
return s.top();
}

/** Returns whether the queue is empty. */
bool empty() {
return s.empty();
}
};